In a vehicle brake system, applying a footbrake provides a hydraulic pressure in a master cylinder and the hydraulic pressure is transmitted through brake lines to wheel brakes. Front wheel brakes are typically of the rotating disc and static caliper type, while the rear wheel brakes are often of the drum type. Both disc brakes and drum brakes operate by application of hydraulic pressure to pistons which force friction members into contact with rotating members.
Obviously, a malfunction in transmitting hydraulic pressure to the wheel brakes will create problems. For example, leakage of fluid will cause depression of a brake pedal to become spongy, and excessive leakage will render the brake system inoperable. Air or contamination in the brake fluid will cause similar results. Thus, it is known to provide a dual master cylinder having one chamber for supply of hydraulic pressure to two wheel brakes and having a separate chamber for supply of hydraulic pressure to the other two wheel brakes. Generally, the first chamber operates the front disc brakes and the second chamber operates the back drum brakes, but criss-cross systems are in use. A dual master cylinder brake system includes a combination valve which functions to control the hydraulic pressure from the second chamber, relative to the pressure from the first chamber.
The dual master cylinder establishes a backup brake subsystem, since should one subsystem fail, the other will continue to operate to brake the car. A single subsystem, however, only has approximately sixty percent of the braking power of a properly functioning dual master cylinder braking system. Additionally, both subsystems have a number of mechanical parts in common, from the brake pedal and its linkage, through the master cylinder and the combination valve. Thus, failure of one of these mechanical parts could render the entire brake system inoperable.
Should either a single or a dual master cylinder brake system fail, the only remaining braking power lies in application of the emergency brake. The emergency brake, however, is a mechanical brake, with only approximately ten percent of braking ability of a hydraulic system. Thus, the conventional emergency brake functions best as a parking brake, rather than a means of stopping a moving vehicle.
It is an object of the present invention to provide a dual master cylinder hydraulic system which does not lose a substantial amount of braking power should one subsystem fail and, further, to provide such a system with an emergency brake system which is functional in braking a speeding vehicle.